In HVDC transmission lines, different types of protection mechanisms may be used in order to detect faults over a broad impedance range. For instance, main protection functions may be employed for covering transmission line faults having impedances in the low-impedance range, i.e., up to 20 Ohm. Line differential protections functions, on the other hand, typically provide a higher sensitivity, allowing to cover faults with impedances up to 400 Ohm.
Since known fault protections techniques for HVDC transmission lines have a limited sensitivity, and, hence, are only capable of detecting faults within a limited impedance range, a fault protection mechanism suitable for high-impedance faults is desirable.